This invention relates to an improvement in the process for isolating chloroprene polymers from their alkaline latices. Chloroprene polymers within the scope of this invention include, in addition to homopolymers of chloroprene, also copolymers of chloroprene with up to equal weight of copolymerizable organic monomers.
Polymerization of chloroprene with or without additional monomers usually is carried out in a soap-stabilized, alkaline, aqueous emulsion. Soaps normally used in such polymerizations are the sodium or potassium salts of rosin or modified rosin. The preferred process for isolating the polymer from the latex involves continuous formation of a polymer film by coagulation on a freeze drum, followed by washing of the film and drying, as described by M. A. Youker in Chemical Engineering Progress, Vol. 43, No. 8, pp. 391-398 (1947). The process is also described in U.S. Pat. Nos. 2,187,146; 3,310,544; and 3,311,601. For most efficient coagulation at the freeze drum, the latex is made slightly acidic prior to the freeze-drum step. To prevent premature coagulation of the latex during the acidification, small amounts of acid-stable surfactants should be present in the latex. These either are present in the emulsion during the polymerization or are added afterwards to the latex.
The acidification is carried out with weak acids such as acetic acid. Strong mineral acids are avoided because even at high dilution they tend to coagulate the latex. Premature coagulation of the latex causes the formation of large irregular masses of polymer which eventually make further operation of the process difficult or impossible. The pH of the latex at the beginning of the freeze-drum isolation step is about 5.0-6.5.
Acidification of the alkaline latex with acetic acid gives sodium acetate, which is occluded in part in the coagulated polymer film. A certain amount of this sodium acetate is desirable because it improves the storage and aging stability of the polymer. However, too much sodium acetate is undesirable because it reduces the curability of th polymer. The amount of sodium acetate remaining in the polymer is ordinarily referred to in the industry as the polymer's alkaline reserve. This is determined by titrating a tetrahydrofuran solution of the polymer with aqueous hydrochloric acid to a neutral red end point (pH 6.2-6.4) using methylene blue as indicator. The measured value is usually between 0.6 and 2.2 milliequivalents of HCl per 100 g. of polymer.
Because of the criticality of the alkaline reserve, the chloroprene polymer film which leaves the freeze drum is subjected to a washing step. The efficiency of this step is dependent on the film thickness and tends to decrease as the film thickness increases. This problem becomes even more complicated when low alkaline reserve is desired. It can be readily seen that the entire process for the isolation of the chloroprene polymer from its latex can be limited by the ability of the washing system to remove sodium acetate from the polymer film to the desired level in the given time. If this step is slow, the plant output will be limited; but if the efficiency of the washing step could be improved or another method of obtaining the desired alkaline reserve were found, the production rate could be increased.